Statistical analyses of rock colors and decomposition grades

Abstract

Color is an effective indicator of rock type and states of weathering. This study analysed the relationship between rock color and decomposition grades using corebox photographs and compared the colors logged by geologists with the computer-determined colors regarding color accuracy. Six granite groups, including slightly decomposed and moderately decomposed fine-grained, medium-grained and coarse-grained granites, common rocks in Hong Kong, were chosen for this study. High-resolution corebox photographs were obtained from GEO GInfo and Dr Louis Wong of the University of Hong Kong. Their corresponding color description and rock classification were extracted from drillhole records and reviewed based on the attached photographs. One hundred and twenty numbers of 200x200 pixel sample images were cropped from rescaled and primarily calibrated corebox photographs for each slightly decomposed granite group. Fifty to sixty images were cropped for each moderately decomposed granite group, with a scale of 0.37 mm per pixel. Each 200x200 pixel sample image was further resized to 5x5, 25x25, 125x125 and 200x200 pixel subsample images for analyses of size effect. An RGB color digitising method was developed to obtain considerably accurate color values from each subsample image. The obtained RGB values were presented in tables, histograms, 3D color point models, and 2D scatter plots for comparison, and were expressed qualitatively as color descriptions by converting values from sRGB color space to CIE XYZ color space and the Munsell color system. It is concluded that notable statistical characteristics and graphical differences were present for each granite group, with grain size and decomposition grade being the two significant controlling factors of granite color. The results recommended against using sample images smaller than 25x25 pixels, i.e. 9.25 mm of physical distance. The objective color logging by conversion of three color spaces was demonstrated. Comparing the results obtained from corebox photographs with those obtained from the Anderson Road quarry site by on-site shooting, some unrepresentative samples were analysed to be related to improper photographic practice, limited color calibration and inappropriate light conditions. These findings indicate the need to specify the technical requirements of corebox photographs and further study the effectiveness of using corebox photographs for rock colour analyses.